1. Field of the Invention
The present invention relates to a heat transfer device, and more specifically, to such a device provided with longitudinal vortex generator winglets functioning as means for restricting separation of heat carrier fluid and means for generating longitudinal vortices.
2. Description of Related Art
In general, a heat exchanger for heating or cooling a fluid is provided with a heat transfer tube through which a thermal medium fluid to be heated or cooled is circulated, and the heat exchanger is so arranged that a heat carrier fluid, such as air, is forcedly moved around the tube. The thermal medium fluid in the tube is cooled or heated by heat exchange with the heat carrier fluid through a tube wall of the tube. In such a heat exchanger using gaseous fluid as the heat carrier fluid, a heat transfer performance depends on the thermal resistance of the heat carrier fluid (e.g., air), and therefore, fins in a variety of forms are attached to the tubes for increasing the heat transferable contact area between the tube and the heat carrier fluid as well as improving the heat transfer performance.
For instance, a high-fin-tube type of heat exchanger which has spiral metal fins attached to metal tubes and the tubes disposed in a staggered arrangement or an in-line arrangement, or a fin-tube type or plate-fin-and-tube type of heat exchanger known as a kind of compact heat exchanger are incorporated in thermal medium circuits of various power plants, thermal carrier circuits of air-conditioning systems, cooling water circuits of various internal combustion engines, and so forth.
This kind of heat exchanger cools the thermal medium fluid through the heat transfer tube by heat exchange of the fluid in the tube with the gaseous flow surrounding the tube. The fin increases the heat transferable area of the tube so as to improve the thermal efficiency of heat exchange between the gaseous flow outside the tube and the fluid inside the tube. As such a fin-tube type heat-exchanger intended to improve its heat exchange performance, a heat exchanger provided with a number of dimples or slits formed on the fins, or a heat exchanger provided with cut and elevated parts formed on the fins for improving its heat exchange efficiency are know in the art (Japanese patent laid-open publications Nos. 11-118379, 7-217999, 8-291988, 61-110889 and so forth).
However, even if the heat transfer effect can be augmented by improvement of configuration of the fin, the pressure loss of the gaseous fluid passing through the heat exchanger greatly increases on the contrary. Therefore, it has been understood to be difficult to realize both augmentation of heat transfer and reduction (or restriction of increase) of pressure loss of the gaseous flow by improving the configuration of the fin.
Technique for improving the heat transfer effect of the heat exchanger without increase of the pressure loss of gaseous flow is disclosed in PCT International Publication (PCT Pamphlet) No. WO2003/014649. In this technique, a heat transfer device of the heat exchanger is provided with vortex generator means (Vortex Generator) generating a longitudinal vortex for augmentation of the heat transfer effect and spouting air flow toward a dead water zone behind the heat transfer tube. The vortex generator means is the delta-winglet positioned in close proximity of the tube. The gaseous fluid flowing near the tube is accelerated by the delta winglet and a swirling flow is caused on the rear of the winglet, whereby the heat transfer effect of the heat exchanger is enhanced by means of restriction of separation, reduction of a dead water area and generation of the longitudinal vortex.
The vortex generator means disclosed in PCT International Publication No. WO2003/014649 is constituted from the delta winglets in a pair, which are intended to reduce the separation wake zone behind the tube and cause the longitudinal vortex behind the winglet by means of the gaseous flow getting over the winglet, thereby augmenting the heat transfer effect of the heat exchanger without increasing the pressure loss of the gaseous flow.
In general, if the flow rate of the gaseous flow (the velocity of the flow) is increased for enhancement of the heat transfer performance, the pressure loss of the gaseous flow is considerably increased in association with increase of the Reynolds number of the air flow. Therefore, according to understanding of those skilled in the art, it is difficult to attain both enhancement of the heat transfer effect and restriction of increase in the pressure loss. However, in the heat exchanger with the aforementioned vortex generator means, augmentation of the heat transfer effect is significant in comparison with increase of the pressure loss in a case where the Reynolds number of the gaseous flow is increased. This advantage is considered to be remarkable, and the vortex generator means can exhibit the expected effect in a relatively large-scale heat-exchanger with the gaseous flow rate being set at a relatively high velocity. However, it has been found that the vortex generator means is difficult to exhibit the effective heat transfer effect in a relatively small-scale heat-exchanger with the gaseous flow rate being set at a relatively low velocity. Therefore, it is considered to be difficult to realize a relatively small-scale heat-exchanger, which achieves both enhancement of the heat transfer effect and restriction of increase in the pressure loss, by means of the vortex generator means only in a pair.
In Japanese patent laid-open publications Nos. 61-99097 and 61-91495, heat transfer devices are disclosed in which a plurality of elevated rectangular walls are arranged along a streamwise direction of the gaseous flow. Even if these walls can generate longitudinal vortices, the vortices caused by the respective walls interfere with each other. Therefore, long continuance of the longitudinal vortex cannot be attained, and the heat transfer device is difficult to exhibit the desired heat transfer effect.
It is a purpose of the present invention to provide a heat transfer device which can improve the heat transfer effect in a heat exchanger with the flow rate of the heat carrier fluid being set at a relatively low velocity, while restricting increase of the pressure loss of the fluid flow.